...no longer relies on Manuel's repo hostig a version of "bignum" that actually
contained bignumber.js. This moves bignumber.js internally and removes bignum
from the browser build process. Also adds a bitcore.Bignum that links to the
right thing. In node, browser.Bignum is require('bignum'). And in the browser,
bitcore.Bignum is is now Bignumber.js (same as before ... except bignumber.js
is now inside bitcore).
Generating random numbers properly depends on the platform. The new
getRandomBuffer method does the right thing on the right platform. It will
sometimes fail due to insufficient entropy. The getPseudoRandomBuffer class is
also provided that will never fail, but it is not cryptographically secure and
should not be used for keys.